The present invention relates to a molded ferrite sheet for use in preparing a thin single-layer soft magnetic sintered ferrite substrate, to a thin single-layer soft magnetic sintered ferrite substrate, and to an antenna module for a noncontact IC tag using RFID (Radio Frequency Identification) technology.
In the fabrication of sintered ferrite substrates, a plurality of superposed molded sheets of a ferrite powder or a mixture of a ferrite powder and a resin are sintered at a time. In this case, the resulting sintered ferrite substrates tend to stick or bond to each other and/or to the sintering table on which they are supported during sintering. When the stuck sintered ferrite substrates are peeled from each other or removed from the sintering table, the sintered ferrite substrates may be damaged. In order to prevent the sticking, a releasing powder such as zirconia powder or alumina powder is usually applied to the surfaces of the molded ferrite sheets and the sintering table prior to sintering and removed after sintering. This procedure is very troublesome. In addition, the releasing powder is difficult to remove completely from the sintered ferrite substrates and thus may contaminate the electronic precision components in which the sintered ferrite substrates are used.
For example, In Japanese Patent Application Laid-open (KOKAI) No. 2-305416 (1990) which relates to a ferrite sheet for supporting a ferrite core of a molded ferrite body and preventing deformation thereof during sintering, it is described, in the description of the related art, that alumina powder is customarily provided on a setter as underlying powder to prevent the deformation by shrinkage during sintering. The method in which the ferrite sheet is used to prevent deformation of a ferrite core is low in productivity. In addition, the use of such an additional ferrite sheet is not desirable from the standpoint of cost. The method using an underlying powder, on the other hand, has a problem because, especially when the molded ferrite sheets to be sintered are thin, the obtained sintered ferrite substrates are apt to undulate or break due to physical contact of the substrates with aggregates of the powder formed thereunder during sintering or some other reasons.
In Japanese Patent Application Laid-open (KOKAI) No. 2006-174223, there is disclosed a method for preparing an antenna-integrated magnetic sheet in which a plurality of 2 mm square ferrite pieces are arranged on a sheet substrate and fixedly bonded thereto. Another sheet substrate and an antenna pattern are then placed over the ferrite pieces. This approach is, however, impractical since it is difficult to arrange the ferrite pieces regularly on a sheet substrate in an efficient manner.
When conventional antenna modules used in noncontact IC tags using the RFID technology and so on cannot transmit or receive radio waves when placed in the vicinity of a metal component because a magnetic flux is converted into an eddy current by the metal component. As a countermeasure against this drawback, a method is widely used in which an antenna module having a conductive loop coil is formed in a spiral shape in a plane and a soft magnetic sheet is laminated in parallel to the coil. In recent years, the demand for size reduction of electronic devices such as cellular phones and for high-density mounting of electronic components is increasing. In addition, the need for an antenna module which is thinner and can provide stable communication even if placed in the vicinity of a metal component is increasing more and more.
In Japanese Patent No. 3,728,320, there is described an invention of an antenna module having a loop coil and a magnetic sheet laminate. A capacitor is connected in parallel to the loop coil to tune the antenna module to a desired frequency such as 13.56 MHz prior to installation. However, when the antenna module is incorporated in an electronic device and the electronic device is placed in the vicinity of a metal component, the resonant frequency of the antenna may be changed. This is a major problem in practical applications. In Japanese Patent Application Laid-open (KOKAI) No. 2005-340759, there is described an antenna module having a metallic shield plate. The antenna module comprises a ferrite sheet-laminated magnetic member having a thickness of about 0.5 mm and coated with PET or PPS, and a metallic shield plate attached to the non-communication face of the magnetic member. However, it is difficult to reduce the thickness of an antenna module of this type, and the recent demand for size reduction of electronic devices cannot be satisfied.